The invention relates to a method for blow-molding containers in which a preform, after a thermal conditioning within a blow-mold of a blow-molding machine, is shaped by the influence of blowing pressure into the container, as well as in which a blowing gas is made available from a supply unit.
The invention further relates to an apparatus for blow-molding containers which includes at least one blow-molding station with a blow-mold, and in which the blow-molding station is connected to a pneumatic supply device.
In a container shaping under the influence of blowing pressure, preforms of a thermoplastic material, for example, preforms of PET (polyethylene terephthalate), are supplied to different processing stations within the blow-molding machine. Such a blow-molding machine typically includes a heating device as well as a blowing device in whose area the previously thermally conditioned preform is expanded into a container through biaxial orientation. The expansion takes place by means of compressed air which is conducted into the preform to be expanded. The sequence in such an expansion of the preform in accordance with process technology is explained in DE-OS 43 40 291. The introduction of the pressurized gas mentioned above also includes the introduction of compressed gas into the developing container bubble as well as the introduction of compressed gas into the preform at the beginning of the blow process.
The basic construction of a blow-molding station for the container shaping is described in DE-OS 42 12 583. Possibilities for thermally conditioning the preforms are explained in DE-OS 23 52 926.
Within the apparatus for blow-molding, the preforms as well as the blow-molded containers can be transported by means of different manipulating devices. Particularly useful has been found the use of transport mandrels onto which the preforms are placed. However, the preforms can also be manipulated by other support devices. The use of gripping tongs for manipulating preforms and the use of spreading mandrels, which for providing support can be inserted into an opening area of the preform, are also among the available constructions.
A manipulation of containers with the use of transfer wheels is described, for example, in DE-OS 199 06 438 in an arrangement of a transfer wheel between a blow-molding wheel and a discharge section.
The manipulation of the preforms already described above takes place, on the one hand, in the so called two-stage methods in which the preforms are initially manufactured in a blow-molding method, are subsequently subjected to intermediate storage, and are only later conditioned with respect to their temperature and are blown up into a container. On the other hand, an application of the so called single-stage method in which the preforms are suitably thermally conditioned directly after their manufacture according to blow-molding technology and a sufficient solidification, and are subsequently expanded.
Different embodiments are known in the art with respect to the blow-molding stations used. In blow-molding stations which are arranged on rotating transport wheels, a book-like opening capability of the mold supports can be found frequently. However, it is also possible to use mold supports which are slidable relative to each other or are guided in different ways. In stationary blow-molding stations which are particularly suitable for receiving several cavities for the shaping of containers, typically plates which are arranged parallel relative to each other are used as mold supports.
A supply of the blow-molding station with blowing gas particularly with compressed air, as a rule takes place through one or more compressors. The compressor used is typically provided with a compressor control and a pressed air storage is installed between the compressor and the blow-molding machine. The compressor control controls the compressor in such a way that a predetermined pressure level is maintained in the area of the compressed air storage. The compressor control is typically adjusted in such a way that in the area of the compressed air storage a pressure level of about 38 bar is made available. The blow-molding machine is provided with a compressed air control which reduces the pressure made available by the compressed air storage to a pressure level as it is specifically required for shaping the container.
A significant portion of the running operating costs for the blow-technological manufacture of the containers results from making available the required blowing air. For reducing these costs it is known, for example, to use the blowing air several times and to conduct the used blowing air of a high pressure level on a lower pressure level once again to the container shaping operation. Since such a recycling of blowing air cannot be used in all cases on a substantial scale, by realization of such a recycling alone, the requirements with respect to a significant reduction of the operating costs can still not be met in a completely satisfying manner.
In the already known methods for blowing air recycling, the predominant portion of the compressed air compressed into the containers is also discharged into the atmosphere. When returning used blowing air, starting from a rotating blowing air supply back into a stationary factory part, substantial line lengths occur and a complex pneumatic system is generated.